The present invention relates to a method of fabricating filament-reinforced composite articles and, more particularly, is related to a method for improving the impact load resistance of such articles when they are struck by foreign objects which are large compared to the filament diameters.
Filament-reinforced composites have become increasingly important in the fabrication of objects such as steam or gas turbine engine blades because their anistropic strength characteristics are reasonably well suited to the loadings which such blades experience during engine operation. These properties are a high stiffness, a corresponding high strength and low density. The strength and stiffness are a direct result of the high tensile strength reinforcing filaments and thse properties vary with orientation of the filaments. For example, the greatest tensile strength and stiffness occurs in the direction parallel to the filaments and, for this reason, the blades are fabricated with the filaments extending in the radial direction which corresponds to the axis of greatest stress. In a transverse direction, however, such composites are relatively weak. Such weakness can be overcome in a composite article by orienting the filaments in adjacent layers in different directions. Forming turbine blades in this manner, however, is not desirable because of the obvious reduction in stiffness and strength in the radial direction.
There is, nevertheless, a need for greater transverse strength in turbine blades to resist fracture and possible catastrophic engine damage when large scale objects such as birds are ingested by the engine. When a blade encouners such an object, the load is first applied at its leading edge and progresses across the blade to the trailing edge. High chordwise and radial stressing results. Without some form of reinforcement, the low-ductility and impact stregnth of a composite blade can result in fracture.
The prior art methods of obtaining higher transverse impact strength include cross-plying the filament reinforced tapes, that is laying the filaments up in different directions as described above, or inserting steel or beryllium laminates in the form of foils or wire mesh between the reinforced tapes. As mentioned above, the cross-plying process results in a natural compromise of the radial strength and stiffness that the reinforcing filaments would otherwise provide in a blade. Steel mesh reinforcing is highly reactive with the aluminum foil or matrix material on which reinforcing filaments are commonly mounted to form the tapes. To avoid such reactivity, the diffusion bonding process in which the tapes in a layup are joined together can only be carried out with strict process controls. For example, the temperature to which the layups are heated is restricted and this restriction in turn requires greater pressures. Non-parallel or intersecting filaments when subjected to the greater pressures have the tendency of fracturing at their intersections. Beryllium mesh while less reactive is very expensive and, though it is strong, it is more brittle than other metals such as steel.
It has been determined that the insertion of titanium laminates between thee boron filament tapes significantly improves the impact load resistance of the resulting article in directions parallel to and transverse to the filaments. At the same time, the titanium and aluminum on which the filements are supported do not react as violently as steel and aluminum so that severely limiting controls need not be placed on the diffusion bonding process in which the tapes and laminates are joined. While titanium has been utilized in a sheath around a filament reinforced blade for erosion and corrosion protection as indicated in U.S. Pat. No. 3,699,623 having the same assignee as the present invention, it is not known to interleave titanium in a filament-reinforced matrix for improved impact load resistance.
It is accordingly a general object of the present invention to provide a method for fabricating a filament-reinforced article in which titanium is included in the filament matrix to improve impact load resistance.